Security systems may be employed to detect an intruder in an environment under surveillance. Examples of security systems include systems using laser light, in which an intruder is detected by a break in a laser light caused by the intruder. Disadvantages of these security systems include high false alarm rates, danger to the eyes due to the laser light, and easily detectible coverage areas. In addition, these system can be easily defeated, for example, when the intruder moves around the laser light. Other systems use a heat sensor to detect the heat emitted by an intruder. These systems can be defeated, for example, by an intruder wearing thermally insulating material. In addition, these systems cannot detect when objects that do not emit detectable heat are removed or added to the environment.
Therefore, there is a need for systems and method capable of detecting changes in a monitored environment that overcome one or more of the disadvantages of current systems.